1. Field of the Invention
The present invention relates to an electroless copper plating solution, more specifically to a copper plating solution used for forming a general copper plating layer including a circuit of a printed board and a conductor of a ceramic thick film substrate.
2. Description of the Related Art
An electroless copper plating solution comprising copper sulfate (CuSO.sub.4) as a copper salt, ethylenediaminetetraacetic acid (EDTA) as a complexing agent for copper ion, and formaldehyde (HCHO) as a reducing agent, etc., is known. In another known electroless copper solution, copper chloride (CuCl.sub.2) is used as a copper salt and Rochelle salt (potassium sodium tartrate) or Quadrol (trade name of N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine) is used as a complexing agent for copper ion. These plating solutions, however, have many drawbacks, including inferior plating layer properties and a very low deposition rate and, thus electroless plating is rarely used for forming a copper plating layer in practice.
Recently, a demand has arisen for a fast electroless copper plating process, to reduce the costs of printed boards, and electroless copper plating baths using an accelerator, baths containing an activator for a reducing agent, etc., have been proposed.
For example, Japanese Unexamined Patent Publication (Kokai) No. 55-65355, published on May 16, 1980, discloses an accelerator having a non-localized .pi.-bond selected from the group of a complex cyclic aromatic nitrogen compound and a sulphur compound, a non-aromatic nitrogen compound having at least one non-localized .pi.-bond, an aromatic amine, and a mixture thereof. An accelerator is also disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-25965, published on Feb. 10, 1984, which comprises 2-mercaptobenzothiazole and imidazole, 4-hydroxypyridine or a mixture thereof. As additives which contribute to an acceleration of the deposition, Japanese Unexamined Patent Publication (Kokai) No. 60-245783, published on Dec. 5, 1985 corresponding to EP 164580 published on Dec. 18, 1985, teaches a cationic polymer containing one or both of acrylic acid and methacrylic acid, and Japanese Unexamined Patent Publications (Kokai) No. 56-271, published on Jan. 6, 1981 corresponding to U.S. Pat. No. 4,303,443 published on Dec. 1, 1981 and No. 57-9865, published on Jan. 19, 1982, teach an ionic surfactant based on polyoxyalkyleneamine. Further, Japanese Unexamined Patent Publications (Kokai) No. 56-272, published on Jan. 6, 1981 corresponding to EP 21757 and No. 60-159173, published on Aug. 20, 1985 corresponding to U.S. patent application Ser. No. 573626, filed on Jan. 25, 1984, disclose a complexing agent for copper ion which contributes to an acceleration of the deposition rate.
However, all of the above proposed plating solutions are unsatisfactory in practice, and therefore, are not used commercially.